diff --git a/lib/carto/torque-reference.js b/lib/carto/torque-reference.js
index 9271b77..3f1227e 100644
--- a/lib/carto/torque-reference.js
+++ b/lib/carto/torque-reference.js
@@ -1047,7 +1047,8 @@ var _mapnik_reference_latest = {
                 "type": "uri",
                 "default-value": "none",
                 "required": true,
-                "doc": "An image file to be repeated and warped along a line"
+                "doc": "An image file to be repeated and warped along a line",
+                "expression": true
             },
             "clip": {
                 "css": "line-pattern-clip",